This used to be somewhat doable, in the sense that you could apply to SSO and HPB and then withdraw from SSO if you matched into HPB (but not the other way around without sitting out a year) but I havent looked recently at the timing of the interview dates, match, withdrawal dates etc to see if it is. Ultimately the thing driving the decision making is that match decisions are binding in the same way that residency match decisions are, so unless the date to withdraw from one match is after the match date for the one you "want more," it will not be possible.
I also dont think its really all that advisable, as although it is true that for the most part, SSO programs and HPB programs are at different institutions (MSK, Toronto and I think MDA are the exceptions to this) its a small world and everyone knows everyone and it looks bad.
If you would like more information on the pros and cons of each approach I would be happy to talk about that, and there are a few videos posted somewhere of lectures given by faculty in each discipline about the topic. It was a major topic of interest for a few years at the various national meetings and so a lot was written about it.
In summary, one-liners for each approach:
HPB: Ideal pathway if you already have a job lined up that specifically just needs you to gain some experience in complex or MIS HPB cases, less than ideal if your goal is a major academic surgonc/HPB job (though not impossible, as I can attest), middle of the road competitiveness (more competitive than transplant, less than surg onc)
Surg Onc: most competitive pathway, ideal if you want to do academic surgical oncology, also ideal if you want to do broad-based community or PP surgical oncology including breast, colons, melanoma, HPB etc
Transplant: Least competitive (though still competitive at the top programs) and the experience in HPB is EXTREMELY variable so approach with caution if the plan is to do HPB for a living, but it opens up the fallback option of doing kidneys and access and the job opportunities in academics are definitely greater in quantity, if not in quality.